1. Field of Invention
The present invention relates to toners for developing an electrostatic latent image, developers for an electrostatic latent image and methods for forming an image employed in electrophotography, electrostatic recording, electrostatic printing and the like. More particularly, the present invention relates to toners for developing an electrostatic latent image, developers for an electrostatic latent image and methods for forming an image using the same for the purpose of developing a digital electrostatic latent image.
2. Description of Related Art
In electrophotography, a toner contained in a developer is deposited onto a latent image formed on a photoconductor and then transferred onto a transfer material such as paper or a plastic film. The toner is then fixed by, for example, heating to form an image. The developer used in this process includes a two-component developer comprising a toner and a carrier and a one-component developer such as a magnetic toner. A two-component developer is widely employed because of its preferable controllability due to the fact that the functions of the developer, such as agitation, transportation and electric charging, are shared with a carrier.
On the other hand, increasing numbers of printers and copiers employing electrophotography have, for the past several years, come to involve a color toning technology and achieved finer electrostatic latent images in response to a higher resolution achieved by improved devices. As a result, accurate development of a latent image and a higher quality of an image have been sought to be obtained by reducing the particle size of a toner. Especially in a full color copier by which a digital image is developed, transferred and fixed using color toners, the quality of an image is increased to some extent by using a small-sized toner having a particle size as small as 7 to 8 .mu.m.
Nevertheless, a further smaller particle size and a more accurate particle size distribution will be required to respond to the recent demand for a higher resolution (improved reproducibility of minute lines, improved gradation, etc.). Reduction in the particle size of a toner is accompanied with increased non-static adhesive forces such as van der Waals force, resulting in an increased cohesive force between toner particles which may lead to a markedly poor particulate flowability or resulting in an increased adhesive force of a toner onto a carrier or a photoconductor surface which may lead to poorer developing and transfer performances, thus causing a reduced image density, which is accompanied occasionally with a marked reduction in ability of cleaning the residual toner on the surface of the photoconductor.
In addition, a reduced charge exchange between the toner and the carrier as a result of a reduced particle performance associated with the reduction in the toner particle size may cause a retarded charging, resulting in a broader charge distribution, which may lead to defects of the image such as fogging. Moreover, the reduction in the particle size of a toner causes a reduced charging performance at a high temperature and a high humidity as well as an evidently retarded charging at a low temperature and a low humidity.
A small-sized toner for full color printing gives a thinner toner layer on a transfer material, thereby requiring a higher concentration of the colorant in the toner. In this case, the charging performance of the colorant contained in the toner is affected more evidently, resulting in a disadvantageously greater difference in electric charge quantity, charging speed, temperature and humidity dependence of the charging between full color toners such as cyan, magenta, yellow and black. This constitutes a considerable problem to be solved. Because of this problem, the formation of a high quality image using a toner having a particle size as small as 6 .mu.m or less has not been established practically.
The thickness of an image formed on a transfer material such as transfer paper (hereinafter referred simply to as "image thickness") is several lm or less in offset printing, but is as large as 10 .mu.m to 20 .mu.m in an electrophotographic process. This is so even when the particle sizes of the toners are as small as 7 to 8 .mu.m because of, for example, the need to form at least three toner layers in the case of the process using full color toners. An image having such a large image thickness tends to exhibit an unusual visual impression. Accordingly, in order to achieve an image of a quality as high as that obtained by transfer printing, it is required to eliminate the difference in the image structure from the transfer printing, i.e., to reduce the image thickness. The image thus formed by mounting a large amount of the toners on the transfer material as described above is readily damaged due to its uneven and irregular surface, resulting in a poor durability of the image once formed.
Accordingly, various attempts have been made to improve full color toners. For example, Japanese Patent Application Laid-Open No. 6-75430, No. 6-332237, No. 7-77824, No. 7-77825 and No. 7-146589 propose a use of a toner whose weight average particle size is 3 to 7 .mu.m, and in which a toner having a particle size of 5.04 .mu.m or less is contained in an amount of 40% by number or more, a toner having a particle size of 4 .mu.m or less is contained in an amount of 20 to 70% by number, a toner having a particle size of 8 .mu.m or more is contained in an amount of 2 to 20% by number and a toner having a particle size of 10.8 .mu.m or more is contained in an amount of 6% by number or less, for the purpose of obtaining an image having a high image density as well as excellent highlight reproducibility and minute line reproducibility.
Japanese Patent Application Laid-Open No. 7-146589 proposes the use of a toner whose weight average particle size is 3.5 to 7.5 .mu.m, and in which a toner having a particle size of 5.04 .mu.m or less is contained in an amount of 35% by number or more, a toner having a particle size of 4 .mu.m or less is contained in an amount of 15% by number or more, a toner having a particle size of 8 .mu.m or more is contained in an amount of 2 to 20% by number and a toner having a particle size of 10.8 .mu.m or more is contained in an amount of 6% by number or less, for the purpose of obtaining an image having a high image density as well as excellent highlight reproducibility and minute line reproducibility.
A small-sized toner discussed in the references listed above has a weight average particle size of the toner particles ranging from 3 to 7 .mu.m, but does not contain toner particles having a size of 5 .mu.m or less in sufficiently large amounts. This allows only a limited improvement in the image quality to be achieved with such a toner. Thus, if such toners are used, there are limits to the improvement in the image quality regarding minute line reproducibility and gradation. Moreover, no discussion is made with regard to the relationship between the amount of the toner having a particle size of 1 .mu.m or less and the characteristics of the toner.
Japanese Patent Application Laid-Open No. 8-227171 proposes a method for imparting excellent transferability and cleanability and for ameliorating the deterioration of toner characteristics due to deterioration of an additive, by means of adding to a toner having a certain form coefficient and a weight average particle size of 1 to 9 .mu.m, a 10 to 90 nm sized inorganic powder and a 30 to 120 nm sized silicon compound microparticle imparted with hydrophobicity.
However, since this toner is combined with an additive having a broad particle size distribution and is not discussed with regard to the rate of the coating onto the toner particle, it cannot be imparted with appropriate particle flowability, particle adhesion ability and electric charging ability when formulated into a toner having a volume average particle size of 5 .mu.m or less, and thus cannot achieve an improved image quality attributable to a small-sized toner. In fact, the weight average particle size of the toner particle described in the examples of this reference is at least 6 .mu.m.
It has also been known to produce toners comprised of polymeric particles impregnated with a dye produced by dispersion polymerization. In this method, the polymeric particle size is perfectly controlled so that all of the particles are of the same size, i.e., there is no particle size distribution. However, this method is used with dyes as colorants and not pigments.
Reduction in the toner size may also lead to difficulty in preserving the electric charge quantity of the toner required for development and in some cases may result in a counter-polarly charged toner. An insufficiently charged toner or a counter-polarly charged toner may cause a blank area in the image or may allow fogging in a non-image region to occur easily. When the electric charge quantity is excessive, the electrostatic adhesion ability becomes too high, resulting in a reduced density or an uneven image structure. Thus, since a smaller-sized toner allows the charging state of an individual toner particle to have a higher effect on the resulting image, it is very important to ensure an appropriate frequency distribution of the electric charge quantity. However, the toners proposed in the references listed above do not discuss the frequency distribution of the electric charge quantity, and practically tend to result in a toner having an insufficient charge, a counter-polarly charged toner and an excessively charged toner, and also still involve the problems of image deterioration such as fogging in a non-image region, a reduced density and an uneven image.
On the other hand, a wet electrophotographic method has been used to avoid the poor qualitative impression of an image by a dry electrophotographic method as described above. The wet electrophotographic method is a procedure in which an image is obtained by developing the image with a liquid developer formed by dispersing a microparticulate toner having an average size of 1 to 2 .mu.m in a carrier fluid such as a petroleum-based solvent having a high boiling point. The method is useful to improve the minute line reproducibility, to reduce the disturbance of the image on a transfer material and to reduce the thickness of an image, thus providing a higher image quality.
Nevertheless, the wet electrophotographic method also involves disadvantages such as reduction in the image quality due to the smeared image, i.e., a toner image on the photoconductor can be distorted by the carrier fluid upon formation of the image forming on the photoconductor. In addition, the method requires a large-sized device which is not suitable for an ordinary office or domestic use, since it must be fitted with a solvent recovery system to avoid the release of the solvents such as the petroleum-based organic solvent having a high boiling point to escape from the instrument. It is undesirable also in view of environmental pollution.
Accordingly, a toner for developing an electrostatic latent image which is applicable to a dry electrophotographic method and which is excellent in terms of minute line reproducibility and stability against environment is sought.
While the problems associated with a conventional small-sized toner are discussed above in connection with the formation of a full color image, a smaller-sized toner is desirable also in the case where an image is obtained in a monochrome system, especially when using only a black toner, since the improved minute line reproducibility and the improved gradation are required similarly and the smaller size of the toner is attributable to improve the image quality also in view of the image thickness.
Also, as a factor for determining the image quality of an image obtained, the surface state of a transfer material appears to be extremely important.
When an ordinary non-coat paper, a high quality paper or copy paper for monochrome printing, etc., is used as a transfer material, there may be a problem that the surface smoothness is insufficient. Moreover, the coloring ability may be decreased as adversely affected by fibers of the adjacent paper when toner particles locate in concave parts of the surface of the paper. Also, the color mixing ability may be deteriorated in the case of secondary colors or tertiary colors. As to the minute line reproducibility, scattering of the thickness may more readily occur and may not be sufficient. In addition, when the toner is not located in the concave parts but instead covers the concave parts but leaves a space in the concave parts, there is an inadequate foundation and thus the toner is not fixed during fixing, and the problem of offset to the fixing roll may occur. In particular, when a small-sized toner is used, the above problems caused by the roughness of the surface state may more easily occur.
When a material having a high surface smoothness such as coat paper is used as a transfer material, since uniform heat and pressure are provided to the toner at fixing, a uniform image having a high glossiness can be obtained. However, if a toner weight per unit area of the toner image on a transfer material is too high, a problem such as spread out of an image at fixing, and a problem such that a glaring image having an excessively high glossiness is obtained and the visual uniformity is decreased, may occur.
In addition, when a material having a paper uniformity and minute unevenness such as mat coat paper, etc. is used as a transfer material, since a toner is fixed to follow the minute unevenness on the surface, the increase of glossiness may be restrained and a uniform image having a low glossiness may be obtained. However, if the toner weight of toner image on a transfer material is too high, the toner existing on the convex is largely molten and glossiness may be increased so that the difference with the glossiness of the transfer material may be increased and the uniformity of image glossiness may be decreased.
As described, there is a problem such that a satisfactory image may not be obtained when a smoothness of the surface of a transfer material is not sufficient. Also, if a toner weight of the toner image on a transfer material is too high, an image having a high uniformity may not be obtained even when the smoothness is high to some extent or sufficiently high.
As a proposal to obtain a high image quality of an image in relation to the surface state of a transfer material and a toner, there is an image forming method by electrostatic copying described in Japanese Patent Application Laid-Open No. 63-123056. In this reference, an image forming method is described in which a toner image developed from an electrostatic latent image using a toner particle which has average radius (ravg) of about 5 .mu.m or less, 90% of the entire of which is in the range from about (0.8.times. ravg) .mu.m to about (1.2.times. ravg) .mu.m and 99% of the entire of which is in the range from about (0.5.times. X ravg) .mu.m to about (2.times. ravg) .mu.m, is transferred electrostatically to the surface of a receiver layer, the surface of which has a peak highness of about (0.3.times. ravg) gm or less. Although it is described that the toner particle may have a size within the range of 1 to 10 .mu.m, it is not indicated whether or not this is on a number average basis or volume average basis. Moreover, in an example in the reference, a dye is used as the colorant instead of a pigment.
With the method, it is described that a low graininess and a high resolution can be attained by corresponding the surface of a transfer material and a profile of the particle size distribution of a toner particle in order to make the adhesive force between the latent image support and toner particles and the adhesive force between the transfer material and toner particles the same, and then applying an electrostatic force in this state to fix.
However, this prior art method cannot be applied to a full-color image formation process requiring a plurality of transfers of toners having different color phases to a transfer material. In addition, in relation with the toner particles to be transferred, the image obtained is largely affected by the surface state of the transfer material, and thus the transfer material to be selected is extremely limited.
Japanese Patent Applications Laid-Open Nos. 5-6033 and Nos. 5-127437 propose a process in which contrarotate developing is made on a non-image region, a transparent toner layer is subsequently formed thereon, a uniform toner layer is formed over the entire of an image region and a non-image region, and the whole of the transfer material surface is smoothed to produce a high gloss image.
However, with the method, the transparent toner amount on the non-image region is 1 to 8 mg/cm.sup.2, compared with the color toner amount on the image region of 0.5 to 5 mg/cm.sup.2. Also, the whole of the transfer material surface is covered by the thick toner layer and thus the transfer material is largely curled. In addition, when the large amount of toner layer is formed on the entire of the non-image region, there is a problem that the consumption amounts of both the color toner and the transparent toner are increased largely, and the cost is thus increased. Further, in these image forming methods, no discussion on the particle size and particle size distribution of toner is made, and thus with the method, the minute line reproducibility and gradation cannot be improved and the image quality obtained is not satisfactory.